One-way gas release valves for maintaining freshness of coffee within a can or other container have long been available. These valves generally are configured to be adhered to the surface of a membrane end that seals the mouth of a coffee container until an end user desires to open the container. The valve covers a small vent hole formed in the membrane end. The valve allows gas pressure within the container to escape when it is higher than ambient pressure while preventing ingress of outside air when ambient pressure is higher than gas pressure within the container. Gas release valves can take on a variety of configurations, but one conventional gas release valve is generally square in shape and includes a base, typically formed of a polyethylene terephthalate (PET) polyester film, having a pressure sensitive adhesive on its bottom face for attachment to the membrane end of a coffee container. A central hole is punched through the base and the pressure sensitive adhesive layer and is sized to surround the small vent hole formed in the membrane end when the valve is applied. A dry strap, which also may be formed of PET film, is wider than the diameter of the hole but narrower than the valve body, and extends across the top surface of the base and covers the hole. A thin film of oil is disposed between the dry strap and the base. An adhesive layer, which may, for instance, be a double sided tape, covers the dry strap and the top surface of the base to secure the dry strap in place atop the hole. A barrier layer, which may be a metalized polyethylene terephthalate (mPET) film, is secured to the opposite side of the adhesive layer forming a cover and protective layer, and helping to bond the dry strap to the base.
In operation, when atmospheric pressure is greater than the gas pressure within the container, the force of the outside pressure urges the dry strap down against the opening in the base. This, in conjunction with the wetting forces provided by the oil film, seals off the opening and prevents air from entering the container through the opening. Gas pressure within the container may become greater than atmospheric pressure due, for instance, to off-gassing from the coffee within the container, a change in barometric pressure, or a change in altitude during shipment. When this occurs, the gas pressure within the container urges the dry strap away from the opening in the base and overcomes the wetting force of the oil, which allows excess gas in the container to flow along the dry strap to the edges of the valve, where it escapes to the atmosphere. In this way, gas pressure within the container is always maintained at or below atmospheric pressure and the container is sealed against ingress of outside air.
In order for the dry strap form a reliable seal around the hole in the base, a very thin film of oil, such as silicone oil, is deposited on the dry strap between the dry strap and the base. This thin film of oil creates a wetting force between the oil and the films of the base and the dry strap and has an inherent surface tension that helps to close and seal the valve when the pressure within the container equalizes with the atmospheric pressure. Traditionally, the oil is applied to the dry strap after the valve is fabricated in a semi-continuous process that involves indexing a long ribbon bearing thousands of valves and micro-spraying milligrams of oil into each valve through the hole in its base. While somewhat successful, this process has proven difficult to control precisely; because, among other things, of the inherent difficulty spraying the correct very small amount of oil into each valve (2.9 milligrams is difficult to control), timing and aligning the spray to the hole in the base, and achieving uniform lay down of oil across the entire dry strap. Variations in these factors lead to corresponding variations in the characteristics of the oil film and consequently in valve performance which, in some cases, can be unacceptable.
An additional and related problem with traditional gas release valves has been that the hole in the base of these valves must be large enough to allow for effective micro-spraying or other application of oil through the hole. However, when the valve is applied to the membrane end of a coffee container, this relatively large hole can allow coffee grinds that migrate through the vent hole in the membrane to become lodged between the dry strap and the base rendering the valve ineffective to seal out ambient air.
Accordingly, there exists an need for a method of making one-way gas release valves for coffee containers with membrane ends, and containers of other commodities, that insures precise application of a tiny volume of oil to each valve, uniform lay down of oil across the entire dry strap, and that eliminates the need to index, align, and apply small volumes of oil through the holes of valves. A further need exists for a one-way gas release valve that is immune to the migration of coffee grinds from a coffee container into the valve. It is to the provision of such a method and resulting valve that the present invention is primarily directed.